Since Andre Geim and Konstanin Novoselof from University of Manchester in UK successfully stripped pyrolytic graphite out and observed graphene in 2004, the investigation of carbon-based new material has been remaining, a hot topic in relevant areas. The success of stripping graphene out breaks the prediction about thermal instability of two-dimensional crystal theoretical, and brings about possibilities for more investigations and explorations in new fields.
Perfect graphene is supposed to own ideal two-dimensional structure, which consists of hexagonal lattice. Every single carbon atom is combined with other three carbon atoms by σ-bond in the direction of lattice plane, and non-bonding electrons serves as π-electrons, forming π-orbit system vertical to the lattice plane in which they could move randomly. But more importantly, due to the unique structure of graphene, the energy band structure shows in the form of the Dirac cone, and graphene conduction and valence bands overlap on the Dirac point. Therefore, effective masses of the electron and cavity on the Dirac point are zero, and the corresponding mobility of electrons and cavity are identical and infinitely close to infinity. This means that the carriers can be either electron or cavity, and the mobility of carriers is large, so the ideal graphene should have excellent conductivity. It is predicted that the current density of graphene is six orders of magnitude higher than that of copper.
At present, a number of exciting research work on large-layer graphene has been published. Meanwhile, two dimensional graphene and graphene oxide particles, the sheet size of which is within Bohr radius, is also investigated intensively. Due to the remaining oxidative functional groups or defect and smaller size, the energy band of such graphene particles is not Continuous. As a result, its carriers can be excited. After being excited carriers form excitons, and the excitons are limited to the band gap of the graphene in the three spatial directions. The thickness of the monolayer grapheme is about 1 nm and the sheet size is around Bohr radius. The band gap of such graphene is large, and therefore the laser emission wavelength formed after deexcitation of the excitons is wider and shows excellent laser characteristics. Such graphene particles are called graphene quantum dots because they have similar characteristics of semiconductor quantum dots in inorganic materials. The radius of graphene quantum dots is within Bohr radius. The quantum dots are non-toxic, with narrow fluorescence wavelength and wide laser wavelength, therefore they can be widely used in the light-emitting diode (LED), biological imaging, photovoltaic devices and sensor.
According to the present research on graphene quantum dots, there are mainly four ways in the preparation of graphene quantum dots. The four methods are as follows: the secondary oxidation based on graphene reoxidation, organic synthesis method starting from small organic molecule, electron or ion beam etching method, and the micro-cutting of the carbon material. Among these methods, second oxidation could not complete subsequent oxidation step well without high quality graphene material. This method is costly and not favorable for industrialization; organic synthesis has complex and tedious process, and is not suitable for industrial production; electron or ion beam etching method has the problem of small yield and could not be adopted for large-scale production; micro-cutting of the carbon material has the disadvantages of complex process and low yield. Therefore, it is imperative to provide a method for large-scale preparation for graphene quantum dots with simple process, which promotes the research progress on quantum dot.